In the past, battery protection devices have been proposed for storage batteries employed in starter circuits of engines. The proposed battery protection devices are directed to solving the frequently occurring problem that arises, for example, when a vehicle operator inadvertently leaves his or her vehicle headlights burning after the engine is stopped, so that the battery is discharged to such a low point that it no longer can start the engine. Such proposed devices are described in U.S. Pat. Nos. 5,200,877; 5,296,788; and 5,296,997, each of which is hereby incorporated by reference.
The basic concept of such battery protection devices is to employ a motor-driven switch in series with the battery that disconnects the battery from the load (for example, headlights) when the energy level of the battery drops to a predetermined level. While the concept of such battery protection devices is sound, implementing the broad concept encounters problems due to high switch contact resistance in the switch-closed position.
In the switch-closed position, high switch contact resistance interferes with the starting of the engine. To start the engine, an engine-starting system containing a battery for powering the starter motor is used. The battery-powered starter motor cranks the engine up to starting speed while overcoming the starting torque of the engine; in so doing, high amounts of current, for example, 500-800 amperes, may be required. Therefore, the cranking speed of the starter motor is highly responsive to the electrical resistance in series with the battery: the internal battery resistance, the electrical resistance of the battery cables, the electrical resistance of connections, as well as the electrical resistance of any switch connected between the battery and starter motor.
For example, if the electrical resistance of such a switch is 15 milli-ohms, and the starter motor requires 600 amperes to achieve cranking speed, the voltage drop across the switch is nine volts, thus reducing the voltage across the starter-motor using a 12-volt battery to less than three volts, the other resistance in the starter circuit causing an additional voltage drop. Measurements made of the electrical resistance across the contact of the switches of prior battery protection devices revealed resistance values as high as 15 to 20 milli-ohms. As shown above, such high levels of electrical resistance could interfere with starting the engine, especially at cold temperatures.
One attempt to solve the above-described problem is to use the polar moment of inertia of a high speed miniature motor, as described in the '877 and '997 patents. Typical motor drive circuitry is arranged to disconnect the electrical drive from the motor immediately upon opening or closing of the contacts using an intrinsic motor limit switch feature. Using the polar moment of inertia can provide a one-half additional revolution after drive power has been removed from the motor to provide greater contact or contact clearance. While this technique provides firm, efficient contact closure and opening, improvements are needed to reduce electrical resistance of the switch contacts.